Thermal spraying is a process of melting and propelling fine particles of molten material such as metal to form a coating. One type of thermal spray gun that has been in use for more than 30 years is a dual wire, arc thermal spray gun, in which two wires are fed into contact at the wire ends that are melted by an electrical arc with current passed through the wires. A jet of compressed gas (usually air) is blown through the tips to atomize (i.e. nebulize) the molten metal and effect a spray stream of molten metal particles. Arc current generally is of the order of hundreds of amperes.
Some applications involve coating inside surfaces of holes or other confined areas such as cylinder bores. For such applications an extension gun is used in which a gas jet from the side deflects the spray from the main axis so that the gun can be inserted into the hole with the angled or deflected spray directed to the surface. The side jet may also act as the primary atomizing jet or be an auxiliary jet to a central atomizing jet. Generally the deflection jet causes a spreading of the spray stream, particularly when such a jet both atomizes and deflects. It is generally known in the art that spreading of the spray stream can result in cooling fringe particles that reduce coating quality, and there has long been a need to reduce this spreading and provide a more constricted spray stream.
Accordingly, an object of the invention is to provide an improved, dual wire, extension type of arc thermal spray apparatus for effecting a narrowed spray stream at an angle to the main axis of the apparatus. Another object is to provide such an apparatus for improving coating quality in confined areas. Yet another object is to provide such an apparatus for improved coating quality without significantly interfering with the arc or atomization for the spray stream. A further object is to provide a novel gas jet member for such an apparatus in order to achieve the foregoing objects.